The role of lysosomes in aortic atherogenesis in White Carneau pigeons was examined by means of acid phosphatase cytochemistry. Foam cells were the major constituent of nascent atherosclerotic lesions in pigeons fed a 0.5% cholesterol diet for either 5 or 10 weeks. Seventy-four percent of foam cell lipid from animals at 5 weeks was in cytoplasmic droplets. The remaining lipid appeared in secondary lysosomes. After 10 weeks of cholesterol feeding, lysosomal lipid accounted for 73% of the lipid volume. The lipid accumulation correlated with increases in both size and number of lysosomes. An average of 2.4 lysosomes per 10(4) cu mu of cytoplasm was observed at 5 weeks. This value doubled by 10 weeks. The average lysosome diameter also increased between 5 and 10 weeks from 2.2 mu to 5.75 mu. Concomitantly, the complexity of lysosomes increased from simple, spherical organelles at 5 weeks to complex, multichambered organelles at 10 weeks. In contrast, lipid storage within cytoplasmic lipid droplets did not change either in size or in number. These observations suggest that by 5 weeks lipid storage within cytoplasmic droplets was maximized, and continued increases in lipid stores occurred predominantly through lysosomal loading.